The interest in optical switching devices has been driven by the tremendous increase in demand for more usage and faster communications systems, i.e. greater bandwidth, in the telecommunication industry. The prime examples of applications that are pushing this demand are the Internet, video/music on demand, and corporate data storage. The existing telecommunication infrastructure, which was largely developed for telephone calls, is now incapable of meeting the demands for new applications of data communication.
Several options have been developed to meet this new demand. These options include wireless, optical, and free-space laser communication technologies. To date, the most promising technology capable of meeting the projected bandwidth requirements of the future is the optical technology.
In an all optical network, or in a combination of an optical and electrical network, the necessary components include a signal carrier medium (i.e. optical fiber), signal routing systems, and data control systems. These signal routing systems have devices which switch optical signals between optical fibers.
In the prior art approaches, the switching of optical signals can be accomplished in predominantly two major approaches: electrical and optical. Today, most systems use electrical switching. In these systems, at the network junctions, the optical signals must first be converted into electrical signals. The converted electrical signals are then switched to the designated channel by integrated circuits. Lastly, the electrical signals must be converted back into optical signals before the signals can be passed onto the optical fiber toward the next destination. Such optical converters are relatively expensive compared to the rest of the transmission equipment.
Electrical switching technology is reliable, inexpensive (except for optical converters), and permits signal reconditioning and monitoring. The main drawback with electrical switching systems is that the number of junctions in a long distance network can be large, and the total cost of converters is very high. Furthermore, typically more than 70% of signals arriving at a junction require only simple straight pass-through, and conversion (down and up conversions) of the full signal results in inefficient use of hardware. System designers also anticipate that future systems are best served by transparent optical switch capabilities; that is, switching systems capable of redirecting the path of the optical signal without regard to the bit rate, data format, or wavelength of the optical signal between the input and output ports. Most electrical switching systems are designed for a specific rate and format, and cannot accommodate multiple and dynamic rates and formats. Future systems will also be required to handle optical signals of different wavelengths, which in an electrical switching network would necessitate the use of separate channels for each wavelength. These limitations of the electrical switching system provide new opportunities for the development of improved optical switching systems.
A switch that directly affects the direction of light path is often referred to as an Optical Cross Connect (OXC). Conventional optical fabrication techniques using glass and other optical substrates cannot generate products that meet the performance and cost requirements for data communication applications. Unlike the electrical switching technique that is based on matured integrated circuit technology, optical switching (ones that can achieve high port count) depends on technologies that are relatively new. The use of micromachining is one such new approach. The term MEMS (Micro Electro-Mechanical Systems) is used to describe devices made using wafer fabrication process by micromachining (mostly on silicon wafers). The batch processing capabilities of MEMS enable the production of these devices at low cost and in large volume.
MEMS-based optical switches can be largely grouped into three categories: 1) silicon mirrors, 2) fluid switches, and 3) thermal-optical switches. Both fluid and thermal-optical switches have been demonstrated, but these technologies lack the ability to scale up to a high number of channels or port counts. A high port count is important to switch a large number of fibers efficiently at the junctions. Thus far, the use of silicon mirrors in a three dimensional (3D) space is the only approach where a high port count (e.g., greater than 1000) is achievable.
Optical Cross Connects that use 3D silicon mirrors face extreme challenges. These systems require very tight angular control of the beam path and a large free space distance between reflective mirrors in order to create a device with high port counts. The precise angular controls required are typically not achievable without an active control of beam paths. Since each path has to be monitored and steered, the resulting system can be complex and costly. These systems also require substantial software and electrical (processing) power to monitor and control the position of each mirror. Since the mirror can be moved in two directions through an infinite number of possible positions (i.e., analog motion), the resulting feedback acquisition and control system can be very complex, particularly for a switch having large port counts. For example, as described in a recent development report, Lucent Technology's relatively small 3D mirror-switching prototype was accompanied by support equipment that occupied three full-size cabinets of control electronics.
Ideally, an optical switch will have at least some of the following principal characteristics:                1) Be scalable to accommodate large port counts (>1000 ports);        2) Be reliable;        3) Be built at a low cost;        4) Have a low switching time;        5) Have a low insertion loss/cross talk.        
While the 3D-silicon mirror can meet the scalability requirement, it cannot achieve the rest of the objectives. Therefore, prior pending patent applications, U.S. patent application Ser. Nos. 09/837,829 and 60/233,672, presented a new approach whereby the complex nature of the 3D free space optical paths and analog control can be replaced with guided optical paths and digital (two states) switching. Such a system greatly simplifies the operation of switching, enhance reliability and performance, while significantly lowering cost. However, there is a need to further improve devices used for switching optical signals because in optical switches, one of the key figures of merit is the Insertion Loss, a parameter that measures the amount of light lost as a result of optical signal traversing through the switch.
The insertion loss consists of a number of components, including loss due to coupling between fiber and switch element, loss due to absorption of light in the waveguide material, and loss due to light traversing in a curved path or around corners. For example, if a waveguide has high-angle bends, there are greater losses in the optical signal passing through the bends. In particular, there is a need to reduce the bending losses in an optical switch element while minimizing the element size. Ideally, the improvement would minimize individual losses and balance the losses between different mechanisms to yield the lowest total insertion loss. In addition to the insertion loss and small element size, other requirements such as power, switching time, and polarization effects are also important considerations in the design.
FIG. 9 is adapted from related and copending U.S. patent application Ser. Nos. 09/837,829 and 60/233,672 and illustrates a concept for using movable microstructure to switching optical signals. In FIG. 9, waveguides 501 are used to conduct optical signals from input 502 to output connections 503. For additional detail, please refer to U.S. patent application Ser. No. 09/837,829. To enable light paths to crossover, waveguide designs with approximately 90-degree bends 504 are shown in FIG. 9. Although a 90-degree bend is possible, such design must be done under numerous constraints; in particular, bend radius. For example, the optical loss due to a waveguide with a bend radius R can be estimated as:             Bend      ⁢                          ⁢      Loss        =          10      ⁢                          ⁢      log      ⁢                          ⁢              exp        ⁡                  [                                    -                              (                                  R                  ⁢                                                                          ⁢                  Θ                                )                                      ⁢                          1                                                kn                  eff                                ⁢                                  a                  2                                                      ⁢                                                            U                  2                                ⁢                                  W                  2                                ⁢                                  ⅇ                                      2                    ⁢                                                                                  ⁢                    W                                                                              1                +                W                                      ⁢                          exp              ⁡                              (                                                      -                                          4                      3                                                        ⁢                                                                                    W                        3                                            ⁢                      Δ                      ⁢                                                                                          ⁢                      R                                                                                      V                        2                                            ⁢                      a                                                                      )                                              ]                    ⁢                          ⁢              (                  in          ⁢                                          ⁢          dB                )              ,where Δ=(n12−n22)/(2n12) is a measure of the difference between the refractive index of the core of the waveguide (n1) and material that surrounds the core (n2). From the equation above, it can be shown that when a small radius is required, it is possible to compensate for loss by using large Δs. Materials with a wide range of refractive indexes have been used successfully in waveguides including silica, silicon, polymer and various other materials.
FIG. 10 illustrates a typical waveguide design where different components of the waveguide are identified. The same material, such as silica, is used for the core 505 as well as for the buffer 506 and cladding 507, but the core is doped with another material to increase its index of refraction. The buffer 506 may be adjacent to a silicon substrate. Using a cladding 507 is not always required since air has an index of refraction (n=1.00) that is lower than any solid material and can be used to guide light effectively.
A main problem with employing large Δs is that the size of the waveguide must be substantially reduced to maintain single mode propagation, which is an important criterion for telecommunication applications. The relationship between waveguide core width for a square waveguide and Δ for single mode propagation is illustrated by the following equation:   d  =      4.272                  kn        1            ⁢                        2          ⁢          Δ                    
As can be seen in above equation, the larger the Δ, the smaller the core size d required. The problem with using small waveguides is that it increases the optical loss due to fiber coupling with a large core fiber. To minimize coupling loss, a lens element is required to match the mode between the fiber and waveguide, which leads to higher manufacturing costs.
A design capable of accommodating large bend radii while maintaining a small size, is highly beneficial to controlling the overall insertion loss. A small size switch element is desirable because more elements can be produced on a single wafer. Small elements also keep the finished size small when they are used in an array connected to form a large port switch.